Make a new bookmark and paste that in for the location. When you come across rot-13 text, select it, then click the bookmark. The decoded text shows up in one of those little javascript boxes. Try it on this: "Qevax hc, rirelobql. Gb gur arj cnegare naq gur byq barf. Jr fvzcyl unir gb trg n fxvashy gb fgnaq gung Xvnevna yhapurba." And of course it works the other way round, as well.